Literature regarding high nitrogen containing stainless steel teaches about the demand for high manganese contents, usually above 5% by weight, in order to increase nitrogen solubility. In order to reduce nickel content, even higher amounts of Mn are recommended. High nitrogen, low nickel wrought stainless steels with contents above 10% Mn are often mentioned in literature and exist commercially.
Compressibility is an important property in PM technology and is a limiting factor when designing an alloy. As high additions of Mn remarkably reduce compressibility, this is not considered an option when using the PM technique. It is also important for the components to have good green strength after compression, in order for the parts not to break during production. Water atomized powder are preferred because they greatly outperform gas atomized powders in this aspect, thanks to the irregular shape of the particles.
Today there are four types of stainless steels represented in the PM industry. Martensitic stainless steels: Typical grade—410. Fe—Cr alloy with low chromium content and generally high strength and hardness.
Ferritic stainless steels: Typical grades 430, 434 Fe—Cr alloy with Cr content 18% by weight, some grades stabilised by Mo or Nb. These steels generally possess high corrosion resistance in air at temperature up to 650° C., low resistance against electrochemical corrosion and medium mechanical properties.
Austenitic stainless steels: Typical grades 304, 316, 310. Fe—Cr—Ni alloys contain from 17 to 25% Cr and from 10 to 20% of Ni, by weight. Some grades contain Mo for improving pitting resistance in quantity up to 6 wt % (e.g. grade Cold 100) These steels generally possess austenitic structure, excellent corrosion resistance but low mechanical properties when sintered in pure hydrogen. Mechanical properties of these steels can be improved by sintering in dissociated ammonia atmosphere (grades 316N1, 316N2, 304N1, 304N2 according MPIF standard No 35), but corrosion resistance will be decreased in this case, because of Cr2N formation during cooling. The other drawback for these steels is their high cost because of the high amount of Ni needed for stabilising austenitic structure and Mo-content to improve pitting resistance.
Duplex grades: Typical grade 17-4. Fe—Cr—Ni alloys contain from 17 to 20% Cr and 3 to 5% Ni, by weight. These steels possess high mechanical properties and medium corrosion resistance.
It is known from U.S. Pat. No. 4,240,831 and U.S. Pat. No. 4,350,529 that corrosion resistance of the 300 series austenitic stainless steels, sintered in nitrogen containing atmosphere can be increased by additional alloying of the powder by elements, selected from the group: Sn, Al, Pb Zn, Mg, rare earth metals, As, Bi. According to these patents stated metals decrease the amount of surface silicon oxides on the powder surface and thereby improve corrosion resistance. Tin is mentioned in literature as an addition that improves corrosion resistance of standard stainless steel grades. It is believed that tin addition decreases the Cr content close to the particle surface which helps to prevent Cr2N formation during cooling in nitrogen containing atmospheres. U.S. Pat. No. 4,420,336, U.S. Pat. No. 4,331,478 and U.S. Pat. No. 4,314,849 all concern tin additions to standard PM stainless steel powder grades to improve corrosion properties. However, neither these patents nor U.S. Pat. No. 4,240,831 or U.S. Pat. No. 4,350,529 teach about stainless steels with nickel contents below 11.2 wt %.
The use of high cooling rate for sintering standard 300 series stainless steel in atmospheres containing nitrogen in quantities up to 25 volume % has been suggested in literature. It is well known that high cooling rates in the temperature range from 1100 to 700° C. prevents Cr2N formation during cooling. However, cooling rates suggested for this purpose are about 195° C./min, which is quite difficult to achieve in the majority of commercially available furnaces.
CN101338385A concerns near full density, high nitrogen, stainless steel products. The products are obtained by subjecting stainless steel powders including 0.1-10 wt % manganese, 5-25 wt % nickel and 0.4-1.5 wt % nitrogen to hot isostatic pressing. All examples in CN101338385A contain above 5 wt % Mn and nickel contents of 9 wt % and above.
Other patents, such as U.S. Pat. No. 6,168,755B1, concern nitrogen alloyed stainless steels produced by nitrogen gas atomization. However, gas atomized powders are less suitable for the press and sintering technique.
U.S. Pat. No. 5,714,115 concerns a low nickel stainless steel alloy with high nitrogen content. However, the manganese content in this alloy is 2 to 26 wt %.
U.S. Pat. No. 6,093,233 concerns a nickel free (less than 0.5 wt %) stainless steel having a ferritic and magnetic structure with at least 0.4 wt % of nitrogen.